Wind tunnels are used to measure the response of a model aircraft to air passing over the model aircraft. The wind tunnel provides a means to evaluate the model aircraft in a controlled environment under conditions that are dynamically similar to conditions to which a full-size version of the model aircraft may be subjected in actual flight. During testing, air in the wind tunnel flows over the model aircraft at controlled speeds in order to evaluate the aerodynamic response of the model aircraft.
The model aircraft may be mounted on an internal balance that may be installed within a cavity in the aircraft fuselage. An aft end of the balance may be coupled to a sting extending out of an aft end of the model aircraft. The sting may be coupled to a wind tunnel support mechanism that provides the ability to statically position and dynamically move the aircraft as air flows over the model aircraft during testing. For example, the sting may change the roll attitude of the model aircraft by rotation about a longitudinal axis of the model aircraft. The sting may also rotate the aircraft about a lateral axis of the model aircraft to change the pitch attitude, or rotate the model aircraft about a vertical axis of the model aircraft to change the yaw attitude of the model aircraft. Changes in roll, pitch, and yaw may be performed in order to measure aerodynamic loads on the model aircraft at different attitudes.
There are six main components of aerodynamic loads that may act on an aircraft, including three forces and three moments. The three forces are normal force, side force, and axial force. Normal force is directed upwardly along the vertical axis, side force acts in a sideways direction along the lateral axis, and axial force acts in an axially aft direction along the longitudinal axis. The three moments include rolling moment, pitching moment, and yawing moment which act as a torque on an aircraft about the respective longitudinal, lateral, and vertical axes of the aircraft.
Measurement of the aerodynamic loads on the model aircraft may be performed using strain gages mounted on the internal balance which may be provided as a multi-piece balance or as a single-piece balance. The strain gages of the internal balance are electrically interconnected in combinations that form bridge circuits for determining the six components of aerodynamic loads on the model aircraft. In general, each bridge circuit generates a bridge electrical response (e.g., measured in micro-volts) in response to an applied load (e.g., measured in engineering units of pounds or inch-pounds). Accurate determination of the magnitude of the loads on a model aircraft is necessary in order to develop and improve the design configuration of a full-size version of the model aircraft. For example, the ability to accurately measure aerodynamic drag on a model aircraft of a commercial airliner in a wind tunnel can result in design improvements that translate into a significant reduction in fuel consumption over the service life of a full-size airliner.
However, of the six components of aerodynamic loads, axial force is typically significantly smaller than the other forces acting on the model aircraft. As a result, axial force sensitivity must be higher than the sensitivity of other loads on the balance, wherein sensitivity may be described as the slope of the bridge electrical response (e.g., in micro-volts) vs. engineering units (e.g., in pounds or inch-pounds). The need for increased axial force sensitivity is complicated by a trend toward increasingly higher static loads and higher dynamic loads on model aircraft in wind tunnels, and which requires balances that have higher static load-carrying capability and higher resistance to fatigue stress for improved fatigue life of the balance. Due to limits on the cross-sectional size of the cavity within the model aircraft, static load-carrying capability and fatigue resistance cannot be improved by increasing the size (e.g., diameter) of the balance.
A multi-piece balance may have a higher load-carrying capability than a single-piece balance of the same diameter, and may therefore be capable of handling the higher stresses associated with increasingly higher static and dynamic loads on a scale model in a wind tunnel. However, the high stresses on a multi-piece balance may result in failure of the balance due to fatigue stress over time. In addition, the quality of data provided by a multi-piece balance generally degrades over time, requiring frequent calibration of the multi-piece balance which is time-consuming, complex, and costly. The relatively lower load-carrying capability of a single-piece balance may require that during wind tunnel testing, loads on the model aircraft are limited to avoid exceeding the structural capability of the balance. Limiting the loads to which the balance is subjected may prevent testing of the model aircraft to its maximum design loads.
As can be seen, there exists a need in the art for a wind tunnel balance having high sensitivity to axial force measurements and which is capable of handling high static and dynamic loads while consistently providing high quality data.